DISCUSSION
In the present study, we found a significant inverse correlation of
LL-37 levels in nasal secretions, with the duration of hospitalization
and the need for medication use in children with acute bronchiolitis.
For the first time, an intriguing association of LL-37 nasal secretion
concentration and disease severity is suggested. To the contrary, no
correlation was found between nasal secretions β-defensin-2 or serum
25(OH)D levels and the severity of bronchiolitis. Finally, there was no
association between AMPs nasal secretions levels and 25(ΟΗ) serum
concentration, age, gender, ethnicity, type of delivery or any
modifiable factor such as breastfeeding and smoke exposure.
Our findings are consistent with limited existing data examining the
association between LL-37 serum levels and the severity of
bronchiolitis. Μansbach et al found a significant inverse correlation
between serum cathelicidin levels, but not 25(OH)D levels, and the risk
of hospitalization >24h in children <2 years old
with bronchiolitis. Serum 25(OH)D concentration or other factors like
age, gender or ethnicity did not correlate with cathelicidin
levels16. A subsequent sizable multicenter cohort
study of infants hospitalized with bronchiolitis confirmed these data,
showing an inverse association between serum LL-37 levels and disease
severity defined by hospital length of stay and intensive care
use17. Finally, this inverse relationship between
LL-37 levels and ICU admission was shown in another prospective cohort
study in infants with Haemophilus-dominant airway
microbiota18. In our study, the lack of association of
nasal secretion LL-37 levels with intensive care use may be attributed
to the small number of children (n=10) admitted to ICU and stringent ICU
admission criteria due to restricted bed capacity.
In our study, AMPs levels in nasal secretions did not correlate with
gender, age, ethnicity, gestational age, birth weight, type of delivery
or smoke exposure. There are few data in the literature concerning
factors that could independently impact AMPs levels in biological
fluids. No correlation was found between serum cathelicidin levels and
race/ethnicity, BMI and body fat percentage in healthy children, but
data concerning age, gender or smoke exposure are
conflicting19-23. Finally, in a study of 115 infants,
cord plasma LL-37 levels were three‐times higher after normal vaginal
delivery compared with Caesarean section24.
According to recent in vitro studies, vitamin D induces the production
of AMPs in a wide range of tissues25 including
bronchial epithelial cells26. However, the question of
whether there is an association between vitamin D and AMPs levels in
vivo is yet to be answered. In the present study, no correlation was
found between 25(OH)D serum concentration and AMPs levels in nasal
secretions. Most of existing studies examine the association between
25(OH)D and AMPs serum levels with conflicting results both in healthy
subjects and patients19,27-30.
In the present study, serum 25(OH)D levels were not associated with the
severity of acute bronchiolitis. These results are consistent with those
of a previous study that found no correlation between vitamin D status
in infants hospitalized for RSV bronchiolitis and disease severity
defined by length of hospitalization, lowest oxygen saturation and
disease severity clinical score31. On the contrary,
there are studies showing an inverse association between vitamin D
levels and the risk of severe bronchiolitis32,33.
Αlthough AMPs seem especially promising therapeutic tools against viral
pathogens, there are certain limitations in their clinical use mainly
due to their pharmacokinetics, as they seem to have low metabolic
stability and undergo degradation by proteolytic enzymes in the systemic
circulation. Consequently, the use of novel techniques that improve AMPs
properties, such as synthetic analogues formation, have generated
increasing interest within the academic and pharmaceutical
communities34-36. Currently, although AMPs can be
administered by various routes, many researchers consider topical
application the only effective way for safe clinical use.
In our study, we showed for the first time an inverse correlation
between LL-37 concentration in nasal secretions and severity of
bronchiolitis. Our findings could have potentially significant
implications if indeed nasal secretion concentration of LL-37 reflect
the impact on lung function closer than serum levels. Interestingly,
Barlow et al showed that treatment of influenza virus infected mice with
nebulised LL-37 reduces disease severity and viral replication,
demonstrating LL-37 antiviral effect in vivo10. Future
controlled randomized trials of local treatment with AMPs via inhalation
hold promise to provide protection against viral lower respiratory tract
infections such as acute bronchiolitis.
There are certain limitations in our study. First, as we have no healthy
control population, serum 25(OH)D and AMPs levels in nasal secretions as
well as the association between them or with other independent factors
was examined only in hospitalized infants with bronchiolitis. Second,
only a small percentage of children with bronchiolitis require
hospitalization, and so the inverse correlation between LL-37 levels and
disease severity may not exist in outpatient infants with
mild-to-moderate illness. Finally, our study does not establish
causality between low LL-37 levels and bronchiolitis severity.
In conclusion, we found a statistically significant association between
LL-37 nasal fluid levels and bronchiolitis severity, independently of
vitamin D status. As we found no correlation between nasal secretion
LL-37 levels with any modifiable factor that could modulate its
endogenous production, we hypothesize that LL-37 concentration in the
respiratory tract mucous membranes itself may predict the progression of
acute bronchiolitis in young children. By inference, if future exogenous
application becomes available, there may be benefits regarding the
severity of disease and length of hospital stay. Since there is not any
effective treatment for acute bronchiolitis yet, the development of
novel alternative therapeutics is of great importance and human LL-37 or
its synthetic analogues can be very promising novel drug candidates in
the future. Therefore, further research is warranted in order to confirm
any beneficial effect of increased LL-37 levels in the nasal secretions
of infants with acute bronchiolitis and determine its predictive value
and potential for clinical applications.